Can't play the game right now, however something struck me.
Considering the fact that you can't shoot outside the circle, wouldn't it take some needless effort to learn when exactly to shoot the enemies?
The idea I'm pitching is that you could make the area outside the circle even more darker to add to the scariness, and once the monsters can be shot inside the circle the whole sprite will glow up a little so you can see that they are actually able to get shot.
As I said, I haven't tried out the game yet but that's my 2 cents for today.